The invention relates to methods and apparatus for measuring in vivo ultrasound attenuation characteristics of limited regions of interest using a pulse echo mode.
The acoustic impedance of a material is the product of its density and the speed of acoustic waves therein. Whenever acoustic energy, such as ultrasound, passes through a boundary defined by a change in acoustic impedance, a portion of the energy is reflected, while the remainder passes through the boundary. In echo ultrasound technology, pulses of ultrasound energy are transmitted into a material for the purpose of producing echo signals for analysis. In medical diagnostics, ultrasound pulses are transmitted into the body. Internal tissue boundaries and inhomogeneities produce reflections of these pulses. The transit time of the reflected energy pulses and knowledge of the speed of sound propagation in tissue permit the determination of the depth of such reflections.
Aside from the echoes which are produced at boundaries where impedance changes, the tissue itself absorbs part of the ultrasound energy. The resultant attenuation of an ultrasound pulse as it passes through tissue is known to be roughly proportional to the frequency of the energy. Similarly structured boundaries and layers, located deeper in the tissue, thus produce weaker signals on account of the absorption of the incident as well as of the reflected signals. The local scattering in tissue varies in an unpredictable manner over small distances. The local attenuation of a single pulse of ultrasound energy is not, therefore, regarded as a meaningful or reproducible diagnostic measurement. Accepted ultrasound imaging techniques therefore rely on imaging boundaries, usually by modulating the intensity of pixels in a display in proportion to the intensity of reflected echoes from corresponding points in the body. Attenuation effects in tissue tend to distort such images. Most imaging systems incorporate some form of time-gain compensation to reduce artifacts attributable to tissue attenuation in the displayed images.
A first prior art technique for measuring ultrasound attenuation required that the material be placed between separate transmitting and receiving transducers. This technique is not considered practical for in vivo measurement in humans since ultrasound can not usually be propagated through the body. A second prior art technique utilized a known reflector to return pulses to a common transmitting/receiving transducer. It is not, however, considered a practical in vivo technique, since it is not usually practical or desirable to insert a reflector into the body. The abovedescribed techniques further measure an attenuation value which characterizes the entire path of the propagating energy and are not suitable for measuring attenuation at a limited region of interest along the path of propagating energy.
A current summary of medical ultrasound imaging, including the discussion of attenuation characteristics and various scan techniques is described generally in an article entitled "Medical Ultrasonic Imaging: An Overview of Principals and Instrumentation" by James F. Havlice and John C. Taenzer, appearing in the Proceedings of the IEEE, Vo.. 67, No. 4, April 1979 which is incorporated herein by reference.
Knowledge of the ultrasound attenuation characteristic within a limited region may have specific value for medical diagnostic purposes. It is known, for example, that diseased livers have different attenuation characteristics than healthy ones. Thus, a measure of attenuation within a region of interest including the liver may provide an indication of a diseased condition which is not otherwise detectable. In addition, attenuation values may be indicative of an origin of a tissue sample and may thus be useful for tissue identification. In tumor detection, the scattering of ultrasound by a tumor may be the same as that of healthy tissue, but the attenuation characteristics may be measurably different. Thus a measure of attenuation will provide additional data which is not utilized in current ultrasound images. Further, since in vivo study of tissue for absorption characteristics is not presently practical, the ability to make depthwise attenuation measurement with ultrasound scanner equipment may provide a further clinical basis for diagnosis.